Microwave reactance tube



Dec. 13, 1955 M. 5. WHEELER MICROWAVE REACTANCE TUBE 2 Sheets-Sheet 1Filed Sept. 20, 1950 XNVENTOR. )VVEfl/V 61 IVA [52515. 233% w ATTORNEY 2Sheets-Sheet 2 M. 5. WHEELER INVENTOR fl /F0 5'. 171955255.

Y fia q ATTORNE MICROWAVE REACTANCE TUBE za'msa' IIIIIIIIIIIIIIIIIIIIIIIDec. 13, 1955 Filed Sept. 20, 1950 United States Patent 12 Claims. (Cl.315-) This application relates to microwave spiral beam reactance tubesof which one beneficial use therefor is in conjunction with radarantenna scanning as more fully set forth in a companion applicationSerial No. 185,758, filed September 20, 1950.

An object of the present invention is to provide an improved reactancetube utilizing an electron stream within a resonant cavity at microwavefrequency to shift the resonant frequency of the cavity.

Another object of the invention is to provide for a large frequencyshift in an electrically efficient manner.

A further object of the invention is to provide, in conjunction with acavity resonator, an electron stream and uniform coaxial magnetic fieldthrough a region of the cavity containing a maximum percentage of thetotal electric energy.

A further object of the invention is to provide an essentially constantelectric field at said region of the cavity containing the maximumpercentage of the total electric energy.

Again, an object of the invention is to provide a structure convenientlycoupling the resonator to the microwave system.

Additionally, an object of the invention is to employ a resonator ofrectangular symmetry and with a relatively short wide region ofinteraction.

At the same time, the invention has an objective of utilizing saidresonator in alignment with the field and with the beam and with thewaveguide input of the frequency being modulated.

Yet another object of the invention is to provide a compact, sturdyconstruction which is readily assembled, contains minimum number ofparts and fabricating operations, and embodies simplicity ofconstruction and operation.

Still further objects of the invention will appear to those skilled inthe art to which it appertains, as the description proceeds, both bydirect reference thereto and by implication from the context.

Referring to'the accompanying drawings in which like numerals ofreference indicate similar parts throughout the several views:

Fig. 1 is an elevation which, in consequence of arbitrary positioning,will be referred to as a plan of a tube embodying my invention;

Fig. 2 is an end view looking from left to right, as at line 60 11- -11of Fig. 1;

Fig. 3 is a longitudinal section on line llIIlI of Fig. 1; Fig. 4 is across-section looking into the cathode end of the tube as at line IVIVof Fig. 3;

Fig. 5 is a longitudinal sectional view as on line VV of Fig. 2;

Fig. 6 is a cross-sectional view looking into the cavity ,resonator frombeneath the cathode as on line VIVI of Fig. 5;

. Fig. 7 isa longitudinal sectional view of a microwave spiral beamreactance tube, modified in construction from the specific showing ofFigs. 1 to 6, and also embodying features of the invention;

Fig. 8 is a similar longitudinal sectional view but taken on aplanetransverse to the plane of section of Fig. 7;

Fig. 9 is a cross section on line IXIX of Fig. 8;

Figures 10 and 11 are sectional views corresponding to Figs. 5 and 6 andshowing a modification of the ribs forming the interaction region.

In the specific embodiment of the invention illustrated in saiddrawings, and with attention directed initially to Figs. 1 to 6, thereference numeral 15 designates a metallic housing here shown asgenerally cylindrical and evacuated. In the particular orientationpresented by the drawings, the right end portion of said housingprovides a large cylindrical chamber 16therein, hermetically sealed byan end cap 17 which is made of iron or other magnetizable metal tofunction, in conjunction with an external magnet 18, as a pole piecewithin the said chamber. Opposite from the metal cap, the chamber isvery considerably restricted by an inner transverse metallic wall 19parallel to the inwardly directed face of said cap. This transverse wall19 has a rectangular cavity 20 therethrough symmetrically disposed withrespect to the axis of the housing, said cavity having a width in onediametric direction of the housing much greater than the height in adiametric direction at right angles to the width, and having a length inthe axial direction of the housing substantially equal to the height andcommensurate with a half wave-length of the characteristic wave forwhich the device is intended. Said rectangular. cavity constitutes aresonator, and to render it more effective as a resonator, the endsthereof in directions axially of the housing are closed except forrestricted or slit openings for passage of electrons, as will presentlybe described.

On inner transverse wall 19 within the chamberv 16 is secured, as byscrews 21, a plate 22-3, preferably of copper, which extends across theend of cavity 23. This plate 22 has a diametric slot 23 partially acrossthe same located symmetricaliy between and parallel to but of lesslength than the long dimensi' ns of width of the resonator. The heightof the slot is considerably less than the height dimension of theresonator, and across said slot 23 are grid Wires or the likeconstituting an accelerator grid 23. Within chamber 16 and close to andparallel with said slot 23 and accelerator grid 23, is an elongatedpreferably flat, electron emissive cathode 24, and between the cathodeand said slot is a control grid 25. The cathode may conveniently be ofthe indirectly heated type, and is shown having heater lead-in wires 26thereto introduced into the chamber 16 by seals 7, and as having alead-in connection 28 thereto introduced through a seal 29 for varyingpotential of the cathode as desired. Similarly a lead-in connection 36is appliec to the control grid and passes to the exterior through a seal31. A rectangular frame is provided in the present showing as means formounting the cathode and control grid and comprises a pair of cleats 3-2on opposite sides of the cathode and control grid, parallel thereto andfixed to the aforementioned plate 22 by brazing or otherwise. Transversemembers 33 are secured to the ends of said cleats, said members beingmade of a suitable insulative material and appropriately apertured toreceive and support end portions of the cathode and grid. By theconstruction shown and described ribbon-like beam of electrons may enterthe resonator through slot 23 and its accelerator grid 23 fromcathode 24under modulationcontrol of grid 25.

7 Within the resonator is provided a restricted interaction region 34constituted by a pair of ribs 35 parallel to the cathode and slot 23 andof substantially the length of said slot so as to be spaced at theirends from the sides of the resonator. These ribs may be integralwith-the housing and project from the;top and bottom of the resonator,and in a direction axially of the housing are of a dimensionconsiderably less than the length of the resonator and located midway ofthat length. The opposed faces of these ribs constitute a condenser sothe ribbon beam of electrons passing therebetween effectively alters thecapacitance of the resonator. As the beam of electrons is under gridcontrol or modulation, resonant frequency is electronically controlledby altering the capacitive reactance of the resonator.

In the construction of tube shown in Figs. 1 to 6, the end of theresonator remote from the entry slot for the beam is provided with awaveguide 36 for transfer of wave energy to and from said resonator.While the exterior of this waveguide is conveniently cylindrical, theinterior is preferably rectangular in cross-section and has dimensionssubstantially the same as the corresponding dimensions of the resonator,namely, a half Wavelength in width. The outer end of this waveguide 36is sealed with a suitable window closure 37. Associated with the sealedmounting of this Window closure is a choke coupling 37a by which thewave-energy is passed through said closure without attenuation.

The inner end of said waveguide is provided with a neck 38 the extremityof which is soldered or otherwise sealed to said housing. Within saidneck is a copper plated iron disc 39 which has a slot 4% diametricallyacross the greater part thereof so the slot width is equal to the majoror larger cross-sectional dimension of the inside of the waveguide,whereby said slot has a width equal to a half wave length. The discfunctions as a fixed pole piece within the tube magnetically coupled toone pole leg of external magnet 18, and the slit therein, which connectslongitudinally between the channel or interior of the waveguide and therectangular cavity 20, functions as a transformer, for which purpose theminor dimension of the slit is less than the narrow or minor dimensionof either the wave guide interior or the resonator. Said slot isparallel to and directly opposite the interaction space of the resonatorand admission slot 23 for the electrons, into the resonator, but with asmaller short or minor dimension than either and such as to give thedesired coupling between the reactance tube and the waveguide. The slitdimension in the direction of the axis of the housing and Waveguide ismade substantially equal to a quarter-wave length. The waveguide is atground potential, and the pole piece or disc 39 also functions as theanode for the electrons from cathode 24.

The housing 15 of the tube and the body of the waveguide are preferablycopper or other non-magnetic metal. Magnetic fiux is caused to traversethe resonator in an axial direction, and as heretofore described, amagnet 18 is provided for that purpose with one pole piece applied to orconstituting the closure for end chamber 16 and the other pole piece 39located within the end of the waveguide at the end of the housing remotefrom the first described pole piece. The magnetic field obtainsspiraling of the electrons which traverse the oscillating electric fieldof the resonator and maintain concentration of the beam in counteractionto the spreading tendency imparted thereto by the cyclic reversal ofpolarities in the afore-mentioned ribs 35.

The window end of the wave uide 35 looks into a magnetron, waveguide orother instrumentality constituting a driving source wherein micro-wavesare present, and in appropriate relation thereto whereby the Wave energyfrom such source will enter said waveguide and induce an alternatingelectric field in said resonator in proper mode. in turn, the resonatoris affected by the electron beam entering from the other side such thatthe efiective capacitance of the resonator is varied in response to gridcontrol of the electron flow and thus the resonant frequency in theresonator is changed and this changed frequency reflects back to thedriving source and changes the frequency thereof. It will be appreciatedfrom the above that the present tube constitutes a reactive load and sois descriptively called a reactance tube and that it is effective with adriving source to apply a modulation thereto or to produce a change ofresonant frequency of the system and obtain tuning to a desiredfrequency.

The specific reactance tube illustrated in said Figures 7 to 9 comprisesa metallic housing which is shown, in the particular orientation of Fig.9, as having the right end portion thereof cylindrical and the left endportion rectangular and permanently attached to the end of thecylindrical portion rather than being shown integral as in Figures 1 to6. Essentially however, whether integral or attached, the portions ofthe housing, as in the previously described construction, in assembledrelation provide a cylindrical chamber 16, hermetically sealed by an endcap 1?" which is made of iron or other magnetizable metal to function,in conjunction with an external magnet 18, as a pole piece within saidchamber. Opposite from the metal cap, the chamber opens into the saidrectangular left end portion of the housing through a slot 23 diametricto the said cylindrical chamber, said slot having accelerator grid 23across the same as previously described. Within said cylindrical chamber16 and parallel to said slot is an electron emissive cathode 24 andcontrol grid 25 both having constructions, locations, mounting,connections and functions in conformity to description above given withrespect to Figs. 1 to 6.

The rectangular portion of the housing beyond said slot 23 isconstructed and arranged to constitute a resonator, and similar to thefirst-described qualifications thereof said resonator provides arestricted interaction region constituted by a pair of ribs parallel tothe cathode and slot 23 and substantially the length of said slot so asto be spaced at their ends from the sides of the resonator. These ribsmay be integral with the rectangular portion of the housing and projectfrom the top and bottom of the resonator, and are of a dimension, in thedirection of electron flow, considerably less than the length of theresonator where the designation of length of the resonator is also thedimension thereof in the direction of electron flow. in thismodification, as well as in the first described construction, the ribsare midway of the said length of the resonator, and the opposed faces ofthe ribs constitute a condenser, such that the ribbon beam of electronspassing therebetween, effectively alters the capacitance of theresonator. As the beam of electrons is under grid control or modulation,resonant frequency is electronically controlled by thus altering thecapacitative reactance of the resonator.

In the showing of Figs. 7 to 9, the Wall of the rectangular housingportion, which is copper or other non-mag netic material, directlyopposite the electron admitting slot 23, constitutes the anode, andsince it is relatively thin, the magnetic field is adequately effectiveby locating the leg of magnet 18 in close proximity to said wall at theout side thereof.

Furthermore, as the physical and electrical require ments for themodification of Figs. 7 to 9 contemplate passage of the driving energydirectly through the resonator in transit from the magnetron to theantenna or other load, the side walls of the resonator are provided withH-transformer slot openings a and 40b in communication with thewaveguide branch from the generator and to the antenna respectively.While the particular branch A is arbitrarily designated in Figs. 7 and8, it will be understood that the same relation of the other branches Bwith a reactance tube identical with the showing of Figs. 7 to 9 isemployed and the drawing is equally illustrative thereof except for theparticular reference character applied to said branch. In the directionof energy passage from the input section of said branch through saidreactance tube to the output section, said H-transformer slot opening40a and 4% have a dimension equal to a quarter wave length of thecharacteristic wave for which the apparatus is designed.

To enable the reactance .tube to be maintained :under .vacuum, windowclosures 37 are ,providedat the junction .of thehranch section of therave guide with ,thevsaidtube,

these window closures beingmounted as in Figures ,1 to 6 andeach havingatchokecoupling37a associated therewith for effectively passing thewave-energy of the guide without imposition of impedance thereto by thewindow closure.

While specific embodiments and usese have been indicated for my improvedreactance tube, reference thereto is by wayrofvexample and is notforrestrictive purposes, Vasthe tube maybe otherwise constructed andutilized within the present inventive concept directed primarily to itsstructure.

From the foregoing description, it is now clear .that an electron streamis used within resonant cavity at microwave frequencies to shift theresonant frequency of the cavity. The requirements have been fulfilledfor a large frequency shift in an electrically efficient manner, therequirements including provision of a cavity resonant at the frequencyof interest; .an electron stream and uniform, coaxial magnetic fieldthrough a region of the cavity containing a maximum percentage of thetotal electric energy; an essentially constant field throughout thisregion of interest and providing interaction with the electron stream;and convenient means of coupling the resonator to the microwave system.These conditions have been met with the resonator herein shown ofrectangular symmetry having, by inclusion of ribs 35, a relativelyshort, wide region of interaction (as related to direction of electronflow). At the same time, an easily machined and calculated waveguideinput can be taken advantage of, by orienting the beam with respect tothe waveguide 36 as shown. The electron stream enters resonator 29 fromthe right'of Figs. 1, 3 and 5, crosses the rectangular interaction space34 in the short direction, and continues therebeyond at constantvelocity and the electrons are collected by the iron pole piece disc 39which possess, in addition to its function as a magnetic pole piece, thefunctions of end wall for the resonator, anode for the electrons, bypredetermined quarter wave length of slot 46 (in direction of electronpath), also functions as a transformer. \Vindow 37 seals the left end ofthe waveguide so the entire interior of the tube may be maintained undervacuum, and that window admits electric driving energy to the resonator.

1n the showing of Figures 10 and 11, the construction of the ribsproviding the interaction region therebetween diifers somewhat from thecorresponding parts in Figures 5 and 6; but that otherwise theconstructions correspond. Essentially, however, both forms shown producean electronically variable impedance at microwave frequencies, bothemploying a static magnetic field and obtaining interaction betweenelectrons and the radio frequency field in a region of uniform magneticfield. The magnetic field has its flux lines substantially am'al with orin the same generally forward direction of the electron transition orbeam path, and the electric radio frequency. field is in a directionperpendicular between the facing faces of the said ribs} In operation,choice is made of an electron velocity for a given magnetic field tominimize the electronic loss in the reactance tube and obtain a largefrequency shift, which is near maximum for that magnetic field.

In Figures 10 and 11, ribs 35a are shown having faces directed towardeach other with gap spacing therebetween constituting interaction region34a. These ribs differ from those of Figs. 5 and 6 in that they have adimension (designated as length herein) in the general direction oftravel of the electron beam equal to the distance from plate 22 to discor polepiece 39 but have a width, transverse to the beam, less than thediameter of the housing 15. This arrangement maintains a relativelyshort region of interaction in the direction of flow of the electronsfor maximum frequency shift but at the same time provides barriers whichestablish a zero field boundary at two ends ,oftheinteraction region. By.this means a cosine electric field distribution is obtained 'in theinteraction region. This has the further beneficial efiect of materiallyreducingv electronic losses whichlare inherent in both types ofconstruction due to theelectron velocity distribution causedby spacecharge. 'These characteristics of the cosine field distribution nullify.very substantially the need for uniform electron veloc'ityacross thebeam.

The construction of Figures 10 and 11 furthermore has the additionaladvantages, that the cavity will have inherently a higher Q, which isgenerally advantageous; that the ratio of energy stored in theinteraction region to the total energy stored in the cavity is increasedand may be uilized to produce a greater frequency deviation; andalsothat the fabrication is less difiicult; all in addition to theadvantage that the reactance tube with .a cosine field distributionthrough the interaction region will produce a larger reactive changewith less losses than .a uniform field tube.

.1 claim:

1. A microwave spiral beam reactance tube comprising a resonator havinga rectangularcavity with two .parallel slot openings at opposite sidesthereof, rectangular parallel ribs in said resonator, said ribs havingwidth, height, length and space therebetween dimensioned to constitutesaid space and interaction region proportionately similar to andcentralized within the cavity of the resonator, a cathode parallel toand opposite said space, an anode opposite said cathode, .said cathodeand anode being on opposite sides of said space for projection of anelectron stream through said space, magnetic poles .oppositely locatedin the direction of the path of the electron stream fromthe cathode tosaid anode, one of .saidfltwoislot openings constituting va transformerslot opening for the resonator, and a window closure beyond saidtransformer slot opening.

2. A microwave spiral beam reactance tube comprising a rectangularresonator the walls whereof have minor dimensions and have longdimensions perpendicular to the minor dimensions, all of the longdimensions of the walls extending in the same general direction, saidresonator having parallel slots at opposite ends thereof and havingrectangular parallel ribs therein on two opposite walls of theresonator, said slots and ribs being in and on'difierent walls of theresonator, said ribs being directed toward each other with a slot-likespace therebetween parallel to and in a common plane with said slots ofthe resonator, said slots, ribs and slot-like space having longdimensions extending in the same general direction as the longdimensions of the walls of the resonator, each of said ribs having itslong dimension substantially equal to the long dimension of the other ofsaid slots, a cathode opposite said one of said slots, and an anodeopposite said cathode, said cathode and anode being on opposite sides ofsaid slot-like space for directing electrons from said one slot to theother through said slot-like space between said ribs.

3. Amicrowave spiral beam reactance tube comprising a rectangularresonator having parallel slots the minor dimension whereof is less thanand in the same general direction as a minor dimension of the resonator,said slots being at opposite ends of the resonator, rectangular parallelribs in said resonator parallel to and spaced from said slots andprojecting toward and normal to a plane common to said slots andproviding a space between said ribs next said plane, said slots, ribsand said space between the ribs having long dimensions thereof extendingin direction of the width of the resonator, each of said ribs having itslong dimension substantially equal to the long dimension of one of saidslots and less than the long dimension of the other of said slots andless than the resonator width, and a cathode opposite said one of saidslots, and an anode opposite said cathode, said cathode and anode beingon opposite sides of said space for directing electrons from said oneslot to the other through said space between said ribs.

4. A microwave spiral beam reactance tube comprising a high Q cavityresonator the cavity whereof has an overall rectangular geometry, ribsin said resonator giving definition to a constricted interaction regiontherebetween of similar rectangular and proportionately smaller geometrythan said cavity and centralized in said cavity for minimum influence ofstatic electric fields and maximum immersion in a uniform staticmagnetic field, a cathode parallel to and as long as the long dimensionof said interaction region, and an anode opposite said cathode, saidcathode and anode being on opposite sides of said interaction region forproducing a flow of electrons across the entire interaction region.

5. A microwave spiral beam reactance tube comprising a high Q cavityresonator the cavity whereof has an over-all rectangular geometry, ribsin said resonator disposed on two opposite walls thereof and each ribhaving direct contact only with the wall on which disposed, said ribsbeing directed with their long dimensions toward each other and inparallelism and giving definition to a constricted interaction regiontherebetween of similar rectangular and proportionately smaller geometrythan said cavity and centralized in said cavity for minimum influence ofstatic electric fields and maximum immersion in a uniform staticmagnetic field, a cathode parallel to and as long as the long dimensionof said interaction region and an anode opposite said cathode, saidcathode and anode being on opposite sides of said interaction region forproducing a flow of electrons across the entire interaction region.

6. A microwave spiral beam reactance tube comprising a high Q cavityresonator the cavity whereof has an overall rectangular geometry, ribsin said resonator giving definition to a constricted interaction regiontherebetween of similar rectangular and proportionately smaller geometrythan said cavity and centralized in said cavity for minimum influence ofstatic electric fields and maximum immersion in a uniform staticmagnetic field, a cathode parallel to and as long as the long dimensionof said interaction region, an anode opposite said cathode, said cathodeand anode being on opposite sides of said interaction region forproducing a flow of electrons across the entire interaction region, andmagnetic means for producing a magnetic field in said interaction regionin the direction of said flow of electrons.

7. A microwave spiral beam reactance tube comprising a high Q cavityresonator the cavity whereof has an overall rectangular geometry, ribsin said resonator disposed on two opposite walls thereof and each ribhaving direct contact only with the wall on which disposed, said ribsbeing directed with their long dimensions toward each other and inparallelism and giving definition to a constricted interaction regiontherebetween of similar rectangular and proportionately smaller geometrythan said cavity and centralized in said cavity for minimum influence ofstatic electric fields and maximum immersion in a uniform staticmagnetic field, a cathode parallel to and as long as the long dimensionof said interaction region, an anode opposite said cathode, said cathodeand anode being on opposite sides of said interaction region forproducing a flow of electrons across the entire interaction region, andmagnetic means for producing a magnetic field in said interaction regionin the direction of said flow of electrons.

8. A microwave spiral beam reactance tube comprising a housing having aresonator therein, a magnetic pole piece and anode constituting one endwall of said resonator, said pole piece having a slot therethrough, andan elongated cathode opposite to and extending parallel to said slot,and a second pole piece, said second pole piece and said cathode beingat the opposite side of the resonator from said slot.

9. A microwave spiral beam reactance tube comprising a housing having aresonator therein, a magnetic pole piece and anode constituting one endwall of the resonator, said pole piece having a slot therethrough, acathode opposite said slot, and a second pole piece, said second polepiece and said cathode being at the opposite side of the resonator fromsaid slot, and a grid between said cathode and resonator.

10. A microwave spiral beam reactance tube comprising a resonator havinga rectangular cross section, a waveguide having a like cross section tothe resonator and alined with said resonator, a magnet pole piece andanode interposed between said resonator and waveguide, a cathode at theopposite end of said resonator from said pole piece and waveguide, and asecond pole piece located at the same end of said resonator as saidcathode.

11. A microwave spiral beam reactance tube comprising a resonator havinga rectangular cross section, a waveguide having a like cross section tothe resonator and alined with said resonator, a magnet pole piece andanode interposed between said resonator and waveguide, and a cathode atthe opposite end of said resonator from said pole piece and waveguide,said resonator and pole piece being of equal dimension in a directionfrom the cathode to the waveguide, and a second pole piece located atthe same end of said resonator as said cathode.

12. A reactance tube having a resonator and a cathode chambercommunicating one with the other, a magnet pole piece closing the outerend of the cathode chamber, a cathode in said cathode chamber, and aslotted disc at the end of the resonator away from the cathode chamber,said disc having a triple function as an end wall for the resonator, asa magnetic pole piece and as an anode.

References Cited in the file of this patent UNITED STATES PATENTS2,410,054 Fremlin et al Oct. 29, 1946 2,466,922 Wax Apr. 12, 19492,523,841 Nordsieck Sept. 26, 1950 2,555,349 Litton June 5, 19512,579,654 Derby Dec. 25,1951 2,591,350 Gorn Apr. 1, 1952

